1. Field of the Invention
The present invention relates to a control system for controlling a parallel hybrid vehicle.
2. Description of the Related Art
Some parallel hybrid vehicles have an internal combustion engine as a primary propulsive power unit and an electric motor for generating an assistive output power for assisting the output power produced by the engine. The output powers (mechanical powers) generated by the engine and the electric motor are transmitted through a power transmitting mechanism including a transmission to drive wheels of the hybrid vehicle.
For accelerating the hybrid vehicle, the electric motor is controlled to generate an assistive output power, and both the output power produced by the engine and the assistive output power from the electric motor are transmitted to the drive wheels. Therefore, the power requirements for accelerating the hybrid vehicle can be met, and the output power produced by the engine may be relatively small, thus reducing fuel consumption by the engine and exhaust gases emitted from the engine.
The electric motor usually comprises a generator motor that can operate also as an electric generator. While the hybrid vehicle is decelerating, the electric motor is operated as an electric generator to regenerate electric energy by the kinetic energy of the hybrid vehicle which is transmitted from the drive wheels through the power transmitting mechanism to the electric motor. The regenerated electric energy is stored in an electric energy storage unit such as a battery used as a power supply for the electric motor. Such an operation is referred to as a regenerative mode of the electric motor.
The transmission of the power transmitting mechanism may comprise an automatic transmission with a torque converter which does not require a clutch between the transmission and the output shaft of the engine or an automatic transmission which requires a clutch between itself and the output shaft of the engine and which is controlled by an actuator.
On a hybrid vehicle with the automatic transmission combined with the clutch, the actuator automatically controls not only the automatic transmission for gear shifts, but also the clutch for engagement and disengagement.
While the hybrid vehicle is running, if the automatic transmission needs to make a gear shift based on a predetermined transmission control map in response to a shift lever operation made by the driver and operating conditions of the hybrid vehicle, then the clutch is disengaged. After the clutch is disengaged, the automatic transmission needs makes the desired gear shift. Then, the clutch is engaged again.
On the above hybrid vehicle, the electric motor may be connected to either the output shaft of the engine at the input side of the clutch or the transmission at the output side of the clutch.
For the automatic transmission to make a gear shift, the clutch is disengaged temporarily. Upon disengagement of the clutch, since no output power of the engine is transmitted to the drive wheels of the hybrid vehicle, the hybrid vehicle runs only under inertial forces. Therefore, when the clutch is disengaged for the automatic transmission to make a gear shift, the driver of the hybrid vehicle feels that the vehicle runs idly.
To avoid such a drawback, it has been customary to control the automatic transmission to make a gear shift as quickly as possible and keep the clutch disengaged as shortly as possible or to engage the clutch as quickly as possible from a disengaged state, i.e., to make a partly engaged state of the clutch as short as possible.
However, there is a certain limitation on efforts to keep the clutch disengaged as shortly as possible. Even if the period of disengagement of the clutch is shortened, only the period of time in which the driver feels that the hybrid vehicle runs idly can be shortened, and such a driver's feeling cannot fully be eliminated.
Furthermore, when the clutch is engaged quickly, the clutch tends to produce a large shock which may be transmitted to the driver. The large shock thus generated may also possibly place an undue burden on the synchromesh mechanism of the automatic transmission, which is liable to be reduced in durability.